Chlorine dioxide is a highly reactive yellowish-green gas that produces useful aqueous solutions in a number of applications such as disinfection, sterilization, and odor control. It is a potent antimicrobial agent, bleaching agent, and as a germicide has found increasing receptivity of its use in municipal and drinking water treatment, cooling towers, and food processing.
Recent regulatory approvals have led to growing acceptance of its use in reducing pathogens in food processing applications such as poultry chill water tanks, beef and pork carcasses washes, and raw agricultural commodities.
Chlorine dioxide has many advantages over traditional chlorine-based biocides due to its higher oxidation potential and greater selectivity towards bacterial cell membranes. However, several drawbacks have limited the implementation of chlorine dioxide where its superior safety and environmental profile would benefit a wide variety of industries.
The biggest disadvantage of using chlorine dioxide is that it must be produced at the-point-of use, i.e. on demand. Chlorine dioxide gas is prohibited from transportation. See 49 CFR 172.101. It is explosive in air at concentrations of about 10% and has a low threshold limit value (TLV) classification by OSHA of 0.1 ppm to workers.
Chlorine dioxide cannot be compressed and stored, as can chlorine, and aqueous solutions of chlorine dioxide rapidly degrade through evaporation and sun light. Aqueous chlorine dioxide chemistry is highly complex and often unwanted side reactions occur which yield lower and higher valent oxy-chloro species such chloride, chlorite, and chlorate. As a result of these limitations, it must be produced at the-point-of use.
The manufacture and transportation of solid chlorine dioxide tablets and powders presents serious safety and stability challenges. The precursors are hydroscopic and deliquescent at high relative humidity, requiring expensive desiccant systems to maintain the relative humidity below 10% in material handling operations. The main ingredient sodium chlorite is combustible in the presence of organic materials at temperatures greater than 150° C. This means that there are maximum allowable limits to the amount of material that can be packaged together in a single package. Furthermore US DOT regulations do not permit the aircraft transport of solid mixtures of activated sodium chlorite (UN 3121 Packing Group I) within the United States.
Chlorine dioxide solid compositions are further disadvantaged in that they are classified as oxidizers and are thus a fire hazard. They must be specially labeled according to UN and DOT regulations and are subject to shipping and storage restrictions. Example hazard ratings of ingredients in various chlorine dioxide release formulations: Sodium chlorite (UN hazard class 5.1-oxidizer); sodium dichloroisocyanurate (UN hazard class 5.1-oxidizer); sodium bisulfate (UN hazard class 8.0-corrosive solid. This disadvantage is discussed in U.S. Pat. No. No. 6,602,442 in column 7, lines 4-58.
A means for building fire retardancy into an oxidizing formulation is discussed in published application WO 2004/089081 A2. The application refers to formulations containing biocide compounds, such as trichloro-isocyanuric acid, which are admixed with an alkaline silicate and boric compounds to form a boro-silicate glass when heated. The boro-silicate glass reduces the combustive properties of the mixture.
An inverse relationship exists between stability and yield, where the highest yield compositions have the lowest stability. High yield compositions such as those described in U.S. Pat. No. 6,699,404 and marketed by BASF under the trademark Aseptrol® must be individually packaged at very low humidity (<10% relative humidity) to prevent moisture from prematurely releasing chlorine dioxide. The presence of humidity in the air prematurely releases chlorine dioxide gas as evidenced by the odor of chlorine dioxide and the ‘yellowing’ of the product or packaging material. Additionally, the composition in US '404 contains sodium dichloroisocyanurate an organic chlorine donor, which contributes to the low temperature stability of this formulation.
U.S. Pat. No. 6,699,404 describes solid chlorine dioxide releasing “massive bodies,” which comprise a mixture of granular particulate ingredients where the size of the particles is substantially smaller than the size of the massive body. The massive body is formed from the mixture of particulate ingredients by compression and is essentially a large tablet. The tablets releases chlorine dioxide when added to water.
U.S. Pat. No. 6,602,442 describes a “dry composition” comprising lithium hypochlorite, sodium chlorite, and sodium hydrogen sulfate. Although this mixture was found to very soluble and rapidly yield chlorine dioxide upon addition to water, a substantial amount of chlorine gas is undesirably released since chlorine is produced above its solubility in water. See column 2, line 55-56 of this patent. Furthermore, the stability of the dry mixture is limited, especially in high humidity environments. These limitations would limit the addition of large amounts to water since the excess chlorine production could cause the mixture to “flash”.
Current water treatment methods employing chlorine dioxide require that the precursors be activated at high concentration, such as 1000 to 3000 ppm, in a smaller container and then diluted into a larger volume of water to yield a desired working concentration, such as 0.5 to 50 ppm. However, the high concentration of chlorine dioxide generated in these methods is difficult and dangerous to handle. This problem is discussed in U.S. Pat. No. 6,602,442 at column 7, line 63 to column 8 line 12. Furthermore, if the precursors are added directly to a large volume of water in one step, it was found that the final concentration was unpredictable and significantly below what would be expected if complete conversion of chlorite to chlorine dioxide took place.
Solid-based chlorine dioxide compositions have been reviewed in U.S. Pat. No. 6,699,404 and WO07078838A3 which are incorporated herein by reference.
There is a need in the industry for a high yield chlorine dioxide composition that is resistant to moisture and heat. A high yield formulation is defined in this application to be where the active components of the composition (excluding binders, fillers, etc.) will release at least 10% of their mass as chlorine dioxide.
The formulation should be stable enough to allow tablets and or powders to be packaged at ambient moisture and temperature, ie. RH=50% Temp=25° C. The composition should also not contain additional hazardous chemicals such as chlorine donors or oxidizers that contribute negatively to the safety profile. Furthermore, the temperature stability of sodium chlorite is greatly reduced in the presence of organic materials such as sodium dichloroisocyanurate. This also applies to organic-based binders and fillers.
There is a need for a stable, solid composition that produces chlorine dioxide upon contact with water.